1. Field of the Invention
This invention resides in the field of biochemical assays performed in receptacles such as cuvettes or the wells of multi-well plates such as MICROTITER® plates, and particularly in obtaining the assay results by optical excitation of the receptacle contents and detection of the emissions resulting from the excitation.
2. Description of the Prior Art
Fluorescence and other optical signals are widely used in biochemical assays, particularly as a label in distinguishing test species that have demonstrated a sought-after property or characteristic in the assay from those that have not. Assays utilizing optical signals are frequently performed in liquid or fluid media retained in a sample receptacle, and optical measurements are performed either on species suspended in the liquid media or on species adhering to the walls of the receptacle, such as species immunologically bound to the walls of microplate wells or cells plated to the bottoms of cuvettes. Instrumentation in current use for measuring fluorescence include a light source and a lens system for focusing a beam into the sample receptacle, and an optical system for collecting and processing the emission light that results from the excitation. The two frequently interfere with each other, however, resulting in a loss of assay sensitivity.
In one type of excitation and emission detection system, a pierced mirror, which is either flat or elliptical, is used for both directing the excitation light to the sample receptacle and collecting the emission light that is produced. Pierced-mirror systems have limited sensitivity, however, since the need to optimize the collection of the weaker emission light requires compromises that result in loss of much of the excitation light. As a result, these devices suffer from a reduced intensity due to restricted aperture considerations and to the misdirection of a portion of the excitation light.
Other systems use a dichroic mirror to separate the excitation and emission light which are otherwise along a common path. The use of a dichroic mirror simplifies the optical path and instrument layout, but efficient separation of the emission light from the excitation light requires an expensive optical filter and a reduction in the signal light. Dichroic mirror systems are principally used in microscopy where light is abundant, rather than in systems where trace amounts of fluorophore are detected with low levels of emission light.